Distances to Stars

Distances are particularly easy to calculate if we use the parsec as our distance unit. In that case, the distance of a star in parsecs is just

where D is the distance in pc and p is the parallax angle in seconds of arc. For example, Sirius has a parallax angle of 0.38 seconds of arc and thus its distance from the Earth is d = 1/0.38 = 2.6 pc = 8.6 LY. The nearest star (other than the Sun) is the alpha-Centauri system, which has a parallax of 0.76 seconds of arc, corresponding to a distance of 1.315pc = 4.3 LY. Thus, all stars have parallax angles of less than one second of arc.